This invention relates generally to lean burn internal combustion engine exhaust gas aftertreatment systems and methods and more particularly to systems and method for estimating oxygen storage capacity and stored NOx in a lean NOx trap aftertreatment systems.
As is known in the art, the exhaust gas generated by a typical internal combustion engine, as may be found in motor vehicles, includes a variety of constituent gases, including hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx), carbon dioxide, nitrogen (N2), and oxygen (O2). The respective rates at which an engine generates these constituent gases are typically dependent upon a variety of factors, including such operating parameters as air-fuel ratio (xcex), engine speed and load, engine temperature, ambient humidity, ignition timing (xe2x80x9csparkxe2x80x9d), and percentage exhaust gas recirculation (xe2x80x9cEGRxe2x80x9d). The prior art often maps values for instantaneous engine-generated or xe2x80x9cfeedgasxe2x80x9d constituents, such as HC, CO and NOx, based, for example, on detected values for instantaneous engine speed and engine load (the latter often being inferred, for example, from intake manifold pressure).
In order to limit the amount of certain ones of the feedgas constituents that are exhausted through the vehicle""s tailpipe to the atmosphere as xe2x80x9cemissionsxe2x80x9d, motor vehicles typically include an exhaust purification system having an upstream and downstream three-way catalyst. For engines capable of running lean of stoichiometry, a downstream three-way catalyst optimized to store and release NOx is used, referred to as a lean NOx trap. This catalyst stores NOx when the exhaust gases are xe2x80x9cleanxe2x80x9d of stoichiometry and releases previously-stored NOx for reduction to unregulated gases when the exhaust gases are xe2x80x9crichxe2x80x9d of stoichiometry. In this manner, the trap permits intermittent lean engine operation, with a view toward maximizing overall fuel economy, while concomitantly serving to control vehicle tailpipe emissions.
More specifically, in one embodiment, the trap chemically stores NOx during lean-burn operation using alkaline metals, such as barium and/or strontium, in the form of a washcoat, although other elements can also be used. The NOx (NO and NO2) are stored in the trap in the form of barium nitrate, for example. The washcoat also includes precious metals, such as platinum and palladium, which operate to convert NO to NO2 for storage in the trap as a nitrate. The trap""s washcoat typically also includes ceria, whose affinity for oxygen storage is such that, during initial lean engine operation, a quantity of the excess oxygen flowing through the trap is immediately stored in the trap. The amount of stored oxygen is essentially fixed, although it begins to lessen over time due to such factors as thermal degradation and trap aging.
The trap""s actual capacity to store NOx is finite and, hence, in order to maintain low tailpipe NOx emissions when running xe2x80x9clean,xe2x80x9d the trap must be periodically cleansed or xe2x80x9cpurgedxe2x80x9d of stored NOx. During the purge event, excess feedgas HC and CO, which are initially consumed in the three-way catalyst to release stored oxygen, ultimately xe2x80x9cbreak throughxe2x80x9d the three-way catalyst and enter the trap, whereupon the trap""s barium nitrate decomposes into NO2 for subsequent conversion by the trap""s precious metals into harmless N2 and O2. The oxygen previously stored in the trap is also released during an initial portion of the purge event after the HC and CO break through the three-way catalyst.
Each purge event is characterized by a fuel xe2x80x9cpenaltyxe2x80x9d consisting generally of an amount of fuel required to release and convert both the oxygen stored in the three-way catalyst, and the oxygen and NOx stored in the trap. Moreover, the trap""s NOx-storage capacity is known to decline in a generally reversible manner over time due to sulfur poisoning or xe2x80x9csulfurization,xe2x80x9d and in a generally irreversible manner over time due, for example, to component xe2x80x9cagingxe2x80x9d from thermal effects and xe2x80x9cdeep-diffusionxe2x80x9d/xe2x80x9cpermanentxe2x80x9d sulfurization. As the trap""s capacity drops, the trap is xe2x80x9cfilledxe2x80x9d more quickly, and trap purge events are scheduled with ever-increasing frequency. This, in turn, increases the overall fuel penalty associated with lean engine operation, thereby further reducing the overall fuel economy benefit of xe2x80x9crunning lean.xe2x80x9d
In order to restore trap capacity, a trap desulfurization event is ultimately scheduled, during which additional fuel is used to heat the trap to a relatively elevated temperature, whereupon a slightly rich air-fuel mixture is provided for a relatively extended period of time to release much of the stored sulfur and rejuvenate the trap. As with each purge event, each desulfurization event typically includes the further xe2x80x9cfuel penaltyxe2x80x9d associated with the initial release of oxygen previously stored in the three-way catalyst and the trap. The prior art teaches scheduling a desulfurization event only when the trap""s NOx-storage capacity falls below a critical level, thereby minimizing the frequency at which such further fuel economy xe2x80x9cpenaltiesxe2x80x9d are incurred.
Typically, an xe2x80x9coptimalxe2x80x9d operating policy which gives the best trade-off between fuel economy and emissions determines when the purge should be initiated or terminated based on operating conditions. The key variable in executing such an optimal solution is an estimate of the amounts of NOx and oxygen stored in LNT (the internal state of the LNT). The purge is initiated when the mass of stored NOx reaches a threshold, and terminated when the stored NOx is completely depleted. Any deviation from the optimal policy, resulting from estimation errors or control implementation errors may lead to adverse consequences on fuel economy and emissions.
Without on-line emission measurements to determine stored NOx or oxygen, one has to estimate the state of the LNT using models and other measured variables. Most commonly available sensors are the switching HEGO sensor located downstream of the trap, mass air flow rate sensor, etc. Since the switching HEGO sensor is positioned after the LNT, a significant time delay may occur between the HEGO switch signal and effective change of the engine feedgas air-to-fuel ratio. This will lead to HC and CO breaking through the exhaust system and cause emission concerns. To mitigate the effects of the time delay, a model-based strategy can be used to predict the end of purge cycle.
Thus, in the absence of on-line emission measurements, the control of lean burn engine aftertreatment system can be implemented by using models of feedgas emissions and the aftertreatment subsystems, together with measured variables such as mass air flow rate, feedgas and tailpipe relative air-to-fuel ratios, etc. When the ambient conditions (such as humidity, LNT brick temperature, and sulfur effects) change or the engine components age, oxygen storage capacity and NOx amount in the LNT vary significantly, and the performance of the control system that relies on these variables may deteriorate if the accuracy of the estimate deteriorate with the change in hardware.
The inventors have discovered that voltage time variations (trajectories) of a tailpipe HEGO sensor can provide sufficient information to predict oxygen capacity during storage phase and stored NOx during purge phase of a lean NOx trap (LNT). This result can be used to provide accurate prediction of the termination time of purge operation, leading to improved control strategies for LNT operation. Further, more expensive UEGO sensors can be avoided without tangible performance penalty on LNT control.
More particularly, the inventors have developed an estimation algorithm to determine the termination time of purge operation. This will reduce potential HC and CO emission problems, leading to improved control strategies for LNT operation. This finding also indicates that more expensive UEGO sensors can be avoided without tangible performance penalty on LNT control. Also, the need to determine the oxygen storage capacity of the LNT is fulfilled by processing the HEGO sensor signal.
In accordance with the present invention, a method is provided for predicting oxygen capacity of an LNT during storage phase of such LNT and the amount of NOx stored in the LNT during purge phase thereof as a function of the time history of an output signal produced by an exhaust gas sensor disposed in an exhaust of an internal combustion engine. A method is provided for estimating oxygen storage capacity of a lean NOx trap (LNT) used in an internal combustion engine. A sensor is disposed in an exhaust downstream of the LNT. The sensor produces an output signal time history indicating an air-fuel ratio time history of such exhaust. The method includes integrating the output signal over time to determine the amount of oxygen stored in the LNT as a function of time. The method includes evaluating the integrated output signal at a time the voltage indicates the exhaust has switched from a condition rich of stoichiometry to a condition lean of stoichiometry. A relative amount of oxygen stored in the LNT is calculated at the switch time. The calculation is a function of the air-fuel ratio of the exhaust and the sensor output signal. The oxygen storage capacity is calculated as a function of the calculated relative amount of oxygen stored in the LNT at the switch time and the evaluated integrated output signal at the time the voltage indicates the exhaust has switched from rich of stoichiometry to lean of stoichiometry.
In accordance with another feature of the invention, a method is provided for estimating the amount of NOx in lean NOx trap (LNT) used in an internal combustion engine. The engine includes a sensor downstream of the LNT. The sensor produces an output signal time history indicating an air-fuel ratio time history of such exhaust. The method includes calculating an oxygen rate from the LNT as over time and an estimated oxygen storage capacity of the LNT. The calculated oxygen rate is evaluated at a time the voltage indicates the exhaust has switched from a condition lean of stoichiometry to a condition rich of stoichiometry. A NOx release rate is calculated at the switch time, such release rate being a function of the output voltage at the switch time. The amount of NOx in the LNT is calculated as a function of time using as an initial condition the calculate NOx release rate at the switch time.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.